Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: dbishopbliss on December 18, 2023, 11:39:30 am
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I thought this would be pretty obvious, but my readings are far from what I expected so I thought i would ask.
I have a JTM45 style amp built by DaBeck Custom Amplifiers. It has a few extras like the option to switch between solid state and tube rectifiers and the first gain stage for each channel has the triodes running in parallel, but other than that its pretty standard.
I decided to try some KT66 tubes in place of the 5881 tubes it came with. A friend lent me his bias checker (https://www.eurotubes.com/store/pc/bias%20probe%20instructions.htm) which seems to be similar to the one offered by Hoffman (https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Tools&ORDER_ID=106870252). I also connected another meter to measure the voltage on the plate. The plate voltage is 450 VDC but the current measured at around 19mA which is way lower than recommended. I tried adjusting the pot to 35mA but it maxed out at 30mA. So then I put the original tubes back in and moved the pot to the starting point again (marked with a line) and the bias measured to 16mA.
According to the instructions I have read, I thought I could just read the mA on my meter (Fluke 77IV) and I would be good to go. But when I see the bias at 16mA it made me think that perhaps I need to multiply the value by the number of tubes which would be 32mA - a value much closer to what people recommend for 5881 tubes.
Should I be multiplying the value or is the pot just not allowing me to bias properly.
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Just set your meter to read mA, not mV. Multiply the mA reading times the plate voltage. Nothing special about it.
Sounds like you need to make a simple mod to your bias supply. That's as specific as I can be without seeing a schematic.
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I just noticed something... my meter wasn't reading 16mA it was reading 0.16mA. I swapped in my cheap meter and it is reading 35 mA which is where I would expect things to be. Now I'm wondering what happened to my "good" meter. I tested the fuses as they instruct in the manual and everything seems Ok.
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Have you ever used the current function on your meter? It's rare that I actually measure current. The last time I did was several years ago when I wanted to know the total B+ current of my newly built AC-15. Which meter do you have?
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Have you ever used the current function on your meter? It's rare that I actually measure current. The last time I did was several years ago when I wanted to know the total B+ current of my newly built AC-15. Which meter do you have?
This is the first time. I have a Fluke 77 IV. I just re-read the manual and I think I know my problem. You have to press the yellow button to measure DC current. :-/
(https://i.ibb.co/zx8PW0g/Fluke77dcma.png) (https://ibb.co/GpsCfbD)
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I just re-read the manual and I think I know my problem. You have to press the yellow button to measure DC current..
Yeah, that would do it!
Need to pay attention to those little symbols with the solid / dashed / wavy horizontal lines.. that indicate the measurement mode.
And always remember that if it's down to you vs. the instrument, trust the instrument first. You're probably using it wrongly (GIGO principle).